Vacuum degassing apparatus

ABSTRACT

A vacuum degassing apparatus is so constructed that molten steel is circulated or flowed between a ladle and a vacuum vessel through a tube, the molten steel is degassed in the vacuum vessel, the surrounding air is prevented from flowing into the ladle during a degassing operation by placing a cover at the top of the ladle, an inert gas is forced into the ladle for sealing the molten steel from the surrounding atmosphere, and, when necessary, a space between the ladle and the vacuum vessel is sealed by a bellows.

BACKGROUND OF THE INVENTION

The present invention relates to a vacuum degassing apparatus and more particularly to a vacuum degassing apparatus for use in a vacuum siphon degassing process or in a vacuum circulation degassing process.

It has been known that properties of a metal or an alloy can be considerably improved by treating it in a vacuum.

In the vacuum siphon degassing process (DH process), a vessel is evacuated with a tube extending from the bottom of the vessel immersed in a molten steel in a ladle. The molten steel is sucked up by the difference in pressure between the surrounding atmosphere and the inside of the vacuum vessel and is progressively degassed therein. By moving the vacuum vessel up and down the molten steel is repeatedly sucked into and discharged from the vessel to thereby make the degassing of the molten steel proceed.

In the vacuum circulation degassing process (RH process), the vessel is evacuated with two tubes extending from the bottom of the vessel immersed in the molten steel in the ladle. By injecting argon gas from the lower end of one of the tubes the molten steel is sucked into the vacuum vessel and degassed therein. The degassed molten steel returns to the ladle through the other tube.

Thus, the two degassing processes have the feature in common that the molten steel is made to circulate or flow between the ladle and the vacuum vessel and is degassed in the vacuum vessel. The apparatuses heretofore employed in these degassing processes were open to the atmosphere at the top of the ladle and, accordingly, had the disadvantage that gas would permeate into the molten steel in the ladle through the slag and reduce the degassing efficiency. In order to prevent the absorbing reaction of the molten steel with the gas from the atmosphere, it has been proposed that the vacuum vessel and the ladle should be contained within a sufficiently large container which is then either evacuated or filled with, for example, an inert gas prior to degassing treatment. This proposal is, however, not suitable because of the huge volume of the container which will require a high cost to build and a long period of time to replace the air within the container.

Accordingly, it is the object of the present invention to provide a vacuum degassing apparatus for use in a DH process or an RH process capable of economically preventing the reduction in degassing efficiency and effectively performing degassing treatment in a short period of time.

SUMMARY OF THE INVENTION

In the vacuum degassing apparatus for use in a DH process or an RH process according to the present invention, a cover is provided at the top of the ladle and a tube fixed to the bottom of the vacuum vessel extends through an opening of the cover and is immersed partially in the molten steel in the ladle. In order to prevent the penetration of the ambient air into the ladle, an inert gas may be forced through an opening of the cover into the space between the top surface of the molten steel in the ladle and the cover. More preferably, a bellows is attached to the bottom of the vacuum vessel so as to surround the tube and to enclose the gap between the tube and the opening of the cover.

The bellows may be mechanically connected between the vacuum vessel and the cover or may have such a construction that the upper end thereof is fixed to the bottom of the vacuum vessel and the lower end thereof is provided with weights along the peripheral edge of the lower end thereof so that the lower end of the bellows is made to freely contact the top surface of the cover by the weights.

In order to facilitate the attachment and removal of the cover to and from the ladle and to ensure the safety of the operation, a cover mounting and demounting mechanism may be provided on the raising and lowering support for the vacuum vessel. Said mounting and demounting mechanism comprises a winch drive unit at the top of the raising and lowering support, and said unit is connected with the cover by ropes or chains. When said winch unit is driven, the cover is moved up or down through the ropes or chains and attached to or removed from the top of the ladle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic vertical sectional view of an apparatus according to the present invention applied in a mechanism for performing a DH process;

FIG. 2 is an enlarged vertical sectional view of a portion of the apparatus of FIG. 1;

FIG. 3 is a plan view of the cover;

FIGS. 4 and 5 are vertical sectional views of other embodiments of the bellows;

FIG. 6 is a schematic vertical sectional view of the apparatus according to the present invention applied in a mechanism for performing a RH process;

FIG. 7 is an enlarged vertical sectional view of a portion of the apparatus of FIG. 6; and

FIG. 8 is a schematic vertical sectional view of another embodiment of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings certain embodiments of the present invention will be described. In FIG. 1 there is shown a schematic vertical sectional view of the apparatus according to the present invention applied in a mechanism particularly for performing a DH process, in which a ladle 2 filled with molten steel 1 is mounted on a truck 3 and transported thereby to beneath a vacuum vessel 4. A cover 5 is placed on the top of the ladle 2 at an appropriate position during transportation thereof. The vacuum vessel 4 is supported by a raising and lowering support 6. When the ladle 2 comes beneath the vacuum vessel 4, the vessel 4 is lowered until a tube 41 extending from the bottom of the vessel 4 passes through an opening 51 (see FIG. 3) of the cover 5 and is immersed partially into the molten steel 1 in the ladle 2.

FIG. 2 is an enlarged vertical sectional view illustrating the relationship between the ladle 2 and the vacuum vessel 4 of FIG. 1. The cover 5 is a sealing cover for the top of the ladle 2 and is formed by lining an iron plate with a refractory material. In addition to the opening 51 for the pipe 41 of the vessel 4, the cover 5 has, as shown in FIG. 3, a normally-closed opening 52 for sampling the molten steel and an opening 53 for inert gas supply. A bellows 7 is provided between the bottom of the vacuum vessel 4 and the top of the cover 5 to prevent the surrounding air from entering a space 21 over the molten steel within the ladle through the gap between the tube 41 of the vacuum vessel 4 and the opening 51 of the cover 5. Accordingly, the bellows 7 is provided so as to enclose a space between the bottom of the vacuum vessel 4 and the top of the cover 5.

Alternatively, a fire-resistant material such as asbestos cloth may be used to form a bellows 7a having a sufficient thickness and enclosing said space with a sufficient slack as shown in FIG. 4. The bellows 7a may be attached as shown in FIG. 4 by fastening the upper and lower ends thereof using bolts and nuts to a rib 42 provided on the bottom of the vacuum vessel 4 surrounding the tube 41 and to a rib 54 provided on the top of the cover 5 surrounding the opening 51. The bellows 7a may be alternatively provided around the lower end thereof with weights 71 as shown in FIG. 5 so that the lower end of the bellows 7a comes into contact with the top face of the cover 5. The construction of the bellows shown in FIG. 5 can shorten the period of time required for attaching and removing the bellows.

According to the present apparatus having the construction described above, degassing treatment is performed satisfactorily while the vacuum vessel 4 is moved up and down by the raising and lowering support 6 since the cover 5 and the bellows 7 enclose the pipe 41 and the upper space 21 of the ladle 2 so as to prevent the surrounding air from coming thereinto.

In order to better prevent the surrounding air from entering the apparatus, an inert gas such as argon or nitrogen gas may be forced into the upper space 21 through the opening 53 of the cover 5. The inert gas can flow outside through the gap between the tube 41 and the cover 5 and the gap between the top edge of the ladle 2 and the cover 5.

The apparatus according to the present invention is applicable to the mechanism for performing an RH process. FIG. 6 is a schematic representation of the apparatus according to the present invention applied to the mechanism for an RH process. In FIG. 6 and FIG. 7, the component parts corresponding to those of FIG. 1 are denoted by the same reference numerals. Two tubes 41a and 41b attached to the bottom of the vacuum vessel 4 extend downward respectively through openings of the cover 5 mounted on the ladle 2 until they are immersed partially into the molten steel 1 in the ladle 2. Into one tube 41a is supplied an adequate inert gas such as argon gas from a source 8 of inert gas to carry the molten steel in the ladle 2 into the vacuum vessel 4. The other tube 41b serves to return the degassed molten steel from the vacuum vessel 4 to the ladle 2.

In FIG. 7 there is shown in an enlarged view a portion of the construction of FIG. 6. The top of the ladle 2 is sealed, as described hereinabove, by the cover 5. The vicinity of the tubes 41a and 41b is enclosed by the bellows 7a which may be in the construction as shown in FIGS. 2 or 4.

In order to prevent the surrounding air from coming into the apparatus, as described hereinabove, an inert gas such as argon or nitrogen gas may be forced into the upper space 21 of the ladle 2 through the opening 53 of the cover 5.

In order to facilitate mounting and demounting of the cover to and from the ladle it is preferred to provide a cover mounting and demounting mechanism in the raising and lowering support for the vacuum vessel. For example, as shown in FIG. 8, said mechanism may comprise an electric motor 9 and a winch 11 mounted on the top of the raising and lowering support 6, in which the winch 11 is connected with the cover 5 by means of ropes 12. When the ropes 12 are normally provided with sufficient slack, the cover 5 is not moved by the raising and lowering motion of the vacuum vessel 4 during the degassing operation.

In the initial degassing stage according to the DH process, namely during the first several raising and lowering operations of the vacuum vessel, there is a tendency for the molten steel not to flow smoothly into the tube attached to the bottom of the vacuum vessel. Accordingly, in such initial stage, it is preferred not to mount the cover upon the ladle immediately but to stop the cover temporarily above the ladle and to control the raising and lowering of the vacuum vessel 4 while watching the flowing-in and discharging of the molten steel into and from the tube. Such an operation is made possible in the embodiment of FIG. 8. When the cover 5 is mounted on the top of the ladle, splashes of the molten steel sometimes adhere around the opening 51 of the cover to make the passing of the tube 41 therethrough difficult. Such a disadvantage is, however, avoided in the construction of the embodiment of FIG. 8.

The cover mounting and demounting mechanism described above is, as will be readily realized by those skilled in the art, applicable to the apparatus for an RH process.

The operating results of the apparatus according to the present invention will now be described in comparison to the operating results of the hitherto existing apparatus as a reference.

EXAMPLE 1

Asbestos cloths each having the thickness of one millimeter were overlapped with one another in three layers to provide airtightness, provided along the lower peripheral edge thereof with steel balls to form fire-resistant bellows 7 and attached to the bottom of the vacuum vessel 4. The ladle 2 having thereon the cover 5 and containing therein molten steel was moved to beneath the vacuum vessel 4. With the tube 41 extending through the opening 51 of the cover 5 and with the molten steel sampling opening 52 and the inert gas injecting opening 53 closed, the bellows 7 was made to enclose above the cover 5. After it was assured that the lower end of the bellows 7 was in a close contact with the top of the cover 5, the vacuum degassing treatment was carried out.

EXAMPLE 2

A gas pipe was laid from an argon evaporator to the inert gas supplying opening 53 of the cover 5 which was provided as in Example 1. After the atmosphere in the upper space 21 of the ladle 2 was replaced by argon gas, suction of the molten steel by the vacuum vessel was started. The vacuum degassing treatment was carried out while replacing the atmosphere in the upper space by argon gas.

REFERENCE EXAMPLE

In the hitherto existing apparatus having the vacuum vessel, ladle and the molten steel at the same conditions as in Examples 1 and 2, the vacuum degassing treatment was carried out without using the cover, bellows or inert gas.

An example of the degassing effect provided by vacuum degassing apparatus according to the present invention is shown in the following table:

                  Table                                                            ______________________________________                                                         Degassing Efficiency                                           ______________________________________                                         Apparatus according to                                                         the present invention                                                                            105%                                                         (Example 1)                                                                    Apparatus according to                                                         the present invention                                                                            120%                                                         (Example 2)                                                                    Oxygen content in the                                                          upper sealed space of                                                          the ladle was 4%                                                               Prior art apparatus                                                                              100%                                                         ______________________________________                                    

As shown in the above table, the apparatus according to the present invention provides an increase in the degassing efficiency of 5 - 20% over the prior art apparatus.

As heretofore described, the present invention is characterized in that the vacuum degassing treatment is carried out with the upper space of the ladle isolated from the atmosphere outside of it. The present invention provides a significant effect to produce steel of high quality in a short period of time by keeping detrimental gas such as hydrogen, nitrogen and oxygen from penetrating into the molten steel using a mechanism which is economical to manufacture and easy to operate.

While we have shown and described specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims. 

We claim:
 1. An improved vacuum degassing apparatus for degassing molten steel in a vacuum vessel by flowing the molten steel between a ladle and the vacuum vessel through a tube, the improvement comprising:a cover having an opening for receiving said tube therethrough and removably mountable on the top of said ladle; a raising and lowering support provided with a mechanism for supporting said vacuum vessel and raising and lowering the same; and a mechanism provided with a winch drive unit mounted on said raising and lowering support and connected with said cover through ropes or chains, for mounting and demounting said cover to and from said ladle.
 2. An apparatus according to claim 1, wherein said cover is provided with an inert gas supplying opening through which the inert gas from a source of supply thereof is forced into said ladle.
 3. An improved vacuum degassing apparatus for degassing molten steel in a vacuum vessel by flowing the molten steel between a ladle and the vacuum vessel through a tube, the improvement comprising:a cover having an opening for receiving said tube therethrough and removably mountable on the top of said ladle; a bellows surrounding said tube and attached to the bottom of said vacuum vessel, for enclosing the gap between said tube and said opening of the cover; a raising and lowering support provided with a mechanism for supporting vessel and raising and lowering the same; and a mechanism provided with a winch drive unit mounted on said raising and lowering support and connected with said cover through ropes or chains, for mounting and demounting said cover to and from said ladle.
 4. An apparatus according to claim 3, wherein said bellows is formed of a flexible fire-resistant material, and said bellows is attached at the upper end thereof to the bottom of said vacuum vessel and provided along the peripheral edge of the lower end thereof with weights so that the lower end of said bellows is freely contactable onto the top face of said cover.
 5. An apparatus according to claim 3, wherein said cover is provided with an inert gas supplying opening through which the inert gas from a source of supply thereof is forced into said ladle. 